Fixed odd behaviour of straight line undo for separate components.
Fixed odd behaviour of straight line undo for separate components.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/64
R=mgiuca@chromium.org, sarakato@google.com

Review URL: https://codereview.appspot.com/200770043

jackhou-chromium/bitmapper,jackhou-chromium/bitmapper,jackhou-chromium/bitmapper